1. Field of the Invention
The present invention relates to a surface light source device having a light emitting surface, and a display device including the surface light source device, and more particularly to a surface light source device and a display device which can effectively equalize the in-plane distribution of perceived brightness.
2. Description of the Related Art
A surface light source device having a two-dimensional light emitting surface is in widespread use as a backlight which is incorporated, for example, in a liquid crystal display device and illuminates a liquid, crystal display panel from the back (see e.g. JP 2007-227405A). Surface light source devices for liquid crystal display devices are roughly classified into direct-light type devices which have a light source disposed right behind an optical member, and edge-light type devices (also called side-light type devices) which have a light source disposed lateral to an optical member.
In an edge-light type surface light source device, a light source is provided lateral to a light guide plate, and light from the light source enters the light guide plate from the side surface (light entrance surface) of the light guide plate. The light that has entered the light guide plate repeatedly reflects from a pair of the opposing main surfaces and travels in the light guide plate in a direction (light guide direction) almost perpendicular to the light entrance surface. The light traveling in the light guide plate, by the optical action of the light guide plate, is caused to gradually exit the pair of the main surfaces as it travels in the light guide plate. In an exemplary light guide plate, a diffusing component is dispersed in the light guide plate. By changing the travel direction of light by the diffusing component, the light is allowed to gradually exit the light guide plate at varying positions along the light guide direction.
Such a surface light source device has been required to secure a high front-direction luminance and enhance the uniformity of the in-plane distribution of the front-direction luminance. Thus, it has been considered ideal for a surface light source device if the angular distributions of luminance, measured at varying positions on the light emitting surface of the surface light source device, are approximately the same and each have the maximum luminance in the front direction. A display device is expected to display a bright image in the front direction by using such a surface light source device.
Nowadays, with the development of LED and the like which can achieve energy saving and linear lighting, an edge-light type surface light source device is becoming larger and has come to be used in combination with a large-sized display surface. A problem, however, exists in that when a surface light source device which has ever been considered ideal is used, considerable variation can be produced in the in-plane distribution of brightness perceived by a viewer.